Delayed action fuse



NOV. 1944- HENRI-GEORGES DOLL I 2,363,234

DELAYED ACTION FUSE i Filed Oct. 6. 19.41 V 3 She tS- Sheet 1 FIGJ. v FIG. 1A.

INVENTOR flaw/W- 550E656 p04 ATTORNEYS 1944- HENRI-GEORGES DOLL I 2,363,234

DELAYED ACTION FUSE Filed 051. s, 1941 s Sheets-Sheet 2 N F J J5 50 ia Q N 61? ill 11 A TTORNE Y5 Nov. 21, 1944. HENR|.GEO RGES DOLL 2,363,234

DELAYED ACTION FUSE I Filed Oct. 6, 1941 v s Sheets-Sheet 3 FIGJO.

' 57 a $5 7 q p? Q 8 Iii.

ATTORNEYS Patented Nov. 21, 1944 DELAYED ACTION FUSE Henri-Georges Doll, Houston, Tex, assignor to Schlumberger Well Surveying Corporation,

Houston, Tex., a corporation of Delaware Application October 6, 1941, Serial No. 413,762

9 Claims. (Cl. 164 -0.5)

The present invention relates to oil well tools a and is, particularly directed to a gun perforator for perforating oil well casings and the like and to a novel time delay fuse for selectively firing a projectile from a submerged gun, although it is not limited to such use.

Present day projectile firing mechanisms suchas gun perforators and sample takers may be divided into two classes. In the first class, aplurality of guns in the gun firing mechanism are fired selectively and in the second the entire group of guns is fired simultaneously or in sequence once the sequence has been started. The selectively fired guns are generally preferred as they permit the projectiles to be fired individually at the will of the operator. Selective firing is generally accomplished by a relatively'complicated switching mechanism lowered with the gun into the bore hole, or by other arrangements for sending current impulses of. increasingly variable strength to heat the fuse wires directly.

This invention provides a simplified gun perforator which is selectively fired, yet has no moving switching parts in the-gun, and which may be selectively fired without the necessity of supplying varying current impulses to the gun.

A primary object of the invention is 'toprovide ignition means with a time delay for igniting an explosive charge.-

Another object of the invention is to provide ignition means having quite different time delay periods, but containing electrically heated igniters which may have identical electrical properties. v

Another object of the invention is to provide means in projectile firing apparatus of the above character for minimizing current leakage from the energizing conductor or conductors at the open contact caused by the firing of a projectile.

Other objects and features of the present invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:

Figure 1 is a view in section of a conventional type fuse wire which is currently used for igniting an explosive charge in a perforating un;

Figure 1A is a view in section of a fuse wire constructedaccording to the present invention and having thereon a layer of heat insulating material;

Figure 1B is a sectional view of a modified form of fuse wire which has been coated with two or.

Figures 2 and 2A are views in section through a spirally wound fuse;

Figures 3 and 3A are views in cross-section of a spirally wound fuse wire which has been imbedded between heat insulating :members of Figure 4B is a sectional view of a hellcally,

wound fuse wire enclosed in a heat insulating tube similar to those shown in Figures 4 and 4A,,but

. in which an air space is left between the wire and ticularly adapted for use in oil insulating tube;

Figures 5 and 5A are views in section. of explosive cartridges which may be used in a perforating gun and in which explosive materialis disposed within a heat insulating shell sur, rounded by a fuse wire;

Figure 6 is a partial view in longitudinalsection, taken along line 8-6 of Figure 6A, of a perforating gun constructed according to the invention, showing one of a number of barrels and projectiles;

Figure 6A is-a partial view in elevation of the perforating gun of Figure 6 illustrating the manner in which an insulated energizing conductor is mountedin the periphery of the gun;

Figure '1 isa partial view in longitudinal section of, another simplified gun perforator employing a fuse of the type shown in Figures 4, 4A and 4B;

Figure 8 is a partial view in longitudinal section of another type of perforator illustrating the external wiring, insulated projectile, time delay fuse, and the connections between the external wiring and the fuse; and a Figures 9, 10 and 11 are schematic wiring diagrams illustrating various circuits for and various uses of the time delay fuses constituting the present invention.

The time delay fuses of the types. shown in Figures 1, 1A,1B, 2, 2A, 3, 3A, 4, 4A., 4B, 5, and 5A'are adapted to be used in electrically firing explosive charges at a distance. They are par, well tools such as gun perforators andsample takers, for example. In the following description, only their use in connection with gun perforators will be de scribed, it being well within the scope of common engineering knowledge to adapt them in any similar device.

Time delay fuses according to the present invention comprise essentially an electrically heated resistance wire or other conductor of electricity, a charge of powder or other explosive material, and means for delaying the transfer of heat from the resistance wire to the explosive charge. .The heat from the small resistance wire may be transmitted to a relatively large mass of material, such a steel, for example, which must be heated for a considerable period of time before it reaches the powder ignition temperature. However, since this principle is relatively impractical due to the limited space available, it is preferred to surround the wire with a small quantity of heat insulating material, such as varnish, Celluloid, twine, rubber, fabric, varnished cambric,

be more constant if the explosive material 23 is used. Its use is therefore preferred.

In the embodiments illustrated in Figures 3 and 3A, a fuse wire 3|.wound in'the form of a spiral is disposed between heat insulating discs 32 and 33 or 33. The heat from fuse wire 3i must raise the temperature of the disc 33 to the powder ignition temperature before the powder (not shown) is ignited. Disc 33' of Figure 3A is thicker than disc 33 of Figure 3, and hence provides a greater time delay. The diameter of the discs 33 and 33 should always be preferably ceramic material, commercial spaghetti tubing,

or the like, through which-the heat must be transmitted to the explosive charge.

By varying the kind or amount of heat insulating material used, the time delay of ignition may be varied.

If a non-inflammable heat insulating material is employed, it may be raised to the powder ignition temperature without burning. n the other hand, an inflammable heat insulating material will burn and the heat from the combustion assist in raising the explosive charge to its ignition temperature.

Figure 1 shows a fuse wire in its simplest form, comprising a resistance wire 20 connected in a suitable circuit (not shown) by conductors 2| through which current may be passed to heat it to a high temperature. The ignition of the explosive charge takes place as soon as the fuse wire 20 reaches the charge ignition temperature.

Figure 1A illustrates how the simple fuse wire of Figure 1 may be transformed into one suitable for use in a delayed action fuse. This may be accomplished by covering the wire 20 with an insulating material such as a coat of varnish 22, for example. It can be seen that the insulating material 22 will have the effect of impeding the transmission of heat so that, for a wire of same section and same nature and for a current of same intensity, a longer time will be required to ignite the powder surrounding the fuse of Figure 1A than the fuse of Figure 1.

The time delay action may be further increased by placing another coating of heat insulating material 23, such as a layerof fabric, for example, over the coat of varnish 22, as shown in Figure 13. Tests have shown that the powder ignition may be readily delayed from a few seconds to several minutes, depending'upon the kind and amount of heat insulation employed.

Figures 2 and 2A illustrate a modified form of time delay fuse in which a fuse wire 24, wound in the form of aspiral, is encased'in a heat insulating tube 25 and placed in a spiral groove 23 cut into a disc 21 made of heat insulating material. Powder, powder paint (powder dissolved in acetone) or other suitable explosive material 28 is poured or packed into the groove 23 on top of and around the wire 24 and the tube 25. The inner end of the wire 24 is passed through'a hole 29 in the disc 21 and isconnected to a contact plate 30 mounted in the rear face of the disc 21.

When the wire 24 is heated. the heat passes through the tube 25 and after a characteristic time delay, ignites the explosive material 23, which in turn ignites the main body of powder (not shown). The explosive material 23 could obviously be dispensed with if desired. However. since large grain powder is sometimes employed for the main body of powder, in which case the contact area between the heated tube 25 and the powder .would be variable, the time delay will great compared to their thickness.

-Figure 4 illustrates another useful form of the time delay fuse in which a fuse wire 34 is wound in the form of a helix and encased in a heat insulating tube 35. The tube i then disposed in a powder charge (not shown). The tube 35 impedes the flow of heat from the heated wire 34 to the powder charge and consequently causes a delay between the heating of the wire and the ignition of the powder charge. The time delay of the fuse of Figure 4 can be increased in several ways, for example, by increasing the thickness of the heat insulating tube 35 as shown in fuse shown in Figure 43 has the same thickness 7 as the tube 35 of Figure 4, but it has a greater diameter than tube 35 of Figure 4. leaving an air space 35 around the wire 34, through which the heat must be transmitted before passing through the tube 35". Thus the ignition of the powder charge is further delayed.

Figure 5 illustrates a time delay cartridge which may be inserted in a gun. The cartridge comprises a capsule 31 made of Celluloid or other suitable material which is filled with a charge of powder 38. A fuse wire 33 is wrapped around the capsule 31, and its two ends are connected to two contacts 40 and 4i on the capsule 31 by means of which it may be connected to an energizing circuit. If desired, the time delay may be increased by using a capsule 31' having a thicker wall than the capsule 31, as shown in Figure 5A.

In Figures 6 and 6A is illustrated a novel perforating gun in which a conductor for energiz ing the filaments which ignite the explosive charges in the gun is disposed in a groove formed in the periphery of the gun, and connections'to the respective igniting filaments are made through the projectiles in the gun. to Figure '6, 42 designates a gun perforator provided with a gun bore 43 which extends laterally therein and terminates rearwardly in a closed powder chamber 44 of slightly smaller diameter than the diameter of the bore 43. The bore 43 and the powder chamber 44 cooperate to form a shoulder 45 which provides a stop for a projectile 46 inthe bore 43 and prevents the explosive charge 41 in the powder chamber 44 from being crushed by the pressure of the bore hole liquid on the projectile 46. The shoulder 45 is not essential, however, and it may be elimi nated, if desired. If the proper type of explosivecharge and fuse are used, the compression of the explosive charge by the projectile 46 will cause no trouble.

The projectile 46 is electrically insulated from the body of the gun perforator 42 b a sheath 43 of insulating material which may be an oxide Referring layer, a coat of insulating varnish, a Celluloid cup, or any other suitable insulating means. The projectile 48 is retained in place within the bore 48 by means of a rubber stopper 48 which also serves to prevent bore hole liquid from leaking into the powder chamber 44.

Within the powder chamber 44 is disposed a fuse cartridge 60, one end of which engages the rear face of the projectile 46 and the other end of which is grounded to the bodyof the perforating gun at the point 6|. The insulated projectile 48 is connected to an insulated conductor 62 which extends through a hole 68 in the plug 48 and through a groove 64 formed along the surface of the body of the gun perforator 42 to a contact assembly 66 disposed in a cylindrical recess 66 located, for example, in the vicinity of the bore 48.

The contact assembly comprises an insulated container 61 within which is mounted a support 68 having a resistance winding 68 thereon. The

terminals of the resistance winding 69 are connected to the contacts 88 and 8|, respectively,

which are connected in series with a conductor 82 disposed in ahelical groove 63 formed in the periphery of the gun perforator 42. The contact 60 is also electrically connected to a conducting member '64 which normally engages a contact plate 66 mounted on the rear face of a plug 86 inserted or threaded in the recess 66.

The conductor 62 which is connected to the projectile 46 is also connected to the contact plate 66. In this fashion the respective fuses for the explosive charges in the gun 42 may be,con-

nected in parallel with series resistances between them. as will be described in greater detail hereinafter in connection with Figure 9.

' The conductor 62 in the groove 63 is connected at the surface of the earth (Figure 9) in series with a conventional type variable resistance 66, an ammeter 61 and switching means 68 to a source of electrical energy 88, the other terminal of which is connected through a conductor 18 to a ground point 1i at the surface of the earth. As shown in greater detail in Figure 9, the fuses 56, 60', 68" and 68" are connected in parallel with the series resistances 69, 88'. and 58" between them.

To assemble, the resistancewinding '68 having its two ends connected to the contacts 80 and 6|, respectively, is placed in the insulated casing 51 which is then disposed within the recess 68 in the gun perforator 42. One end of the conductor 62 is then passed through an aperture 66a in the plug 66 and is lightly soldered to the contact plate 65. A cartridge containing -the explosive charge 41 and a fuse wire 68 is then placed in the powder chamber 44, as shown in Figure 6. The projectile 46, insulated except at the ends thereof and connected at the forward end to the conductor 52, is pushed into the bore 43 until it is seated against the shoulder 46 at which time its rear face is in engagement with the fuse wire 50. Therubber stopper .49, previously threaded over the wire 52, is pressed into position along with the projectile 48.

When it is desired to shoot the projectile 48, the switching means 68 (Figure 9) is closed at the surface of the earth and current is transmitted from the source of electric energy 89 through the ammeter 61, variable resistance 66,

the conductor 62 and the conductor 62 to the fuse wire 60. This current heats the fuse wire positive pole thereof.

powder chamber 44, thereby impelling the projectile 48 out of the bore 4!.

As'the current continues to flow through the conductor 82 to shoot other similar projectiles in the gun perforator 42, there will be a slight leakage current from the contact member 64 through the bore hole liquid to the body of the gun perforator 42. This leakage current may be reduced by making the hole 88a in the plug 66 relatively small. It can also be decreased by making the plug 66 of rubber such that the hole 88a therein will partially close when the conductor 62 has been pulled out by the firing of the projectile 48.

In addition, one very effective method has been devised for decreasing the leakage current. It has been found that the electrical resistance between the exposed contact member 66 and the ground can be materially increased by maintaining the contact 66 at a positive potential. with respect to ground. This may be readily accomplished when using direct current by grounding the negative pole of the source of electrical ener gy 69 and connecting the conductor 62 to the When this is done the flow of current from the contact member 85 causes a layer of gas to be formed on the surface thereof. The gas thus formed is a good electrical insulator and it reduces the leakage current to a minimum. This method is particularly effective as only a relatively small contact area is exposed to the well fluid. Also, since the member 86 is disposed in the recess 66 behind the plug 66, it is protected from the motion of the fluid in the hole so that the gas film is not disturbed.

In th embodiment illustrated in Figure 7 of the drawings, the gun perforator 42 is provided with a laterally extending gun bore 43 terminating rearwardly in a powder chamber 68 of greater diameter than the bore 43, the rear portion.

4A and 4B of the drawings, one end of which engages the rear face of the projectile 46 and the other end of which is grounded to the plug 10 at the point 15.

The operation of the embodiment shown in Figure 7 is the same a that of the embodiment illustrated in Figure 6 and described above. One advantage of the construction illustrated in Figure 7 is that it permits a larger explosive charge to be employed while maintaining the same length gun bore and the same diameter gun body. This embodiment further illustrates: the use of the time delay fuses of Figures 4, 4A and 4B in connection with a large volume of powder.

Figure 8 illustrates the use of delayed action fuses constructed according to the present invention in a conventional type perforating gun.

68 and ignites theexplosive charge 41 in the 76 Referring to Figure 8, the gun perforator 16 is provided with a laterally extending bore 11 having a projectile 46 therein. The bore 11 communicates rearwardly with a concentric cylindrical recess 18 which is threaded at 19 to receive a hollow cylindrical plug 86, forming a cylindrical powder chamber 8| 88 and the body of the gun perforator 16. The

between the plug plug 88 is provided with holes 82,to facilitate its removal by a spanner wrench and a gasket 88 is provided between it and the body of gun perforator 18 in order to insure a tight joint. Within the chamber BI is disposed an annular cartridge 88 having an explosive charge therein. Secured at the rear end of the projectile 88 is a fuse 88 of the type shown in Figures 2 and 2A of the drawings. One end of the wire 28 in the fuse 88 is connected to the rear face of the projectile 88 and the other end is connected to a conical member 88 which engages the plug 88. The circuit connections are essentially the same as described above in connection with Figures 6 and 6A.

In operation, the wire 28 in the fuse 88 is electricaliy heated by closing the switching means 88 at'the surface of the earth (Figure 8). The heat from the fuse 88 ignites the powder paint 28 in the'groove 28 (Figure 2A) after a characteristic time delay. in turn igniting the powder charge in the cartridge 88 and impelling the projectile 88 through the gun bore 11.

The schematic circuit of Figure 9 illustrates how selective firing may be accomplished with identical cartridges connected in parallel without interrupting the flow oi. current being sent to th perforating gun. and 88" may, or may not, be identical, although as a practical matter it is preferable to have them identical. By way of example, the resistance of each of the fuses 88, 88', 88" and 88" may be two ohms and the resistance of each of the re-- 7 88' is obtained. The same procedure will then be followed until the desired number of projectiles have been fired.

Although the current being supplied to the gun need not be interrupted, variations of current registered by the ammeter 81 may be observed as an indication of the firing of each shot. Any desired method may be employed to report the firing of each shot to the surface. For example, a pair of headphones connected across an inductance in series with the conductor 82 will give a characteristic sound each time a shot is fired. Alternatively, a shock-actuated switch may be located at or near the gun such that a circuit including suitable indicating means is momentarily completed or interrupted by the vibration created by the shooting.

If the variable resistance 88 has a high value, it'will be possible to fire a great number of projectiles without altering either the applied potential or the adjustment of the variable resistor 88. In such case, the various projectiles in the gun perforatorv would fire almost immediately were it not for the delayed action produced in the fuses employed. Obviously the time between successive shots may be increased by successively increasing the amount of heat insulating material on the respective fuses. The same result could also be'obtained by successively increasing the value. of the resistances 88, 88' and 58".

Figure 10 is a schematic wiring diagram illus- The resistors 88, 88' 1 trating the use of series connected delayed action fuses and indicating the manner in which the various projectiles in a gun perforator may be.

selectively fired with a substantially constant interrupted current when fuses with similar electrical characteristics are employed. In this embodiment, the conductor 82 is connected to the dissimilar, series-connected cartridges 81, 88, 88 and 88, the return current path being through the earth. The resistances of the fuse wires 81', 88', 88' and 88' in the cartridges 81, 88, 88 and 88 is relatively low, generally being in the neighborhood of a few ohms, while the resistance 88 is relatively high, such as, for example, fifty ohms or more. The fuse wires 81', 88', 88' and 88' are similar but the thickness of the heatinsulation surrounding each is progressively increased so that they will all be heated to approximately the same temperature when the switch- 2 ing'means 88 is closed, but the powder surrounding' the fuse v8l' will be ignited first, since it has the least insulation surrounding the fuse wire. If desired, the first cartridge 81' may be constructed without anyspecial insulating material surrounding the fuse wire.

A short time after the cartridge 81 has ignited, the powder in the cartridge 88 will then be heated to its ignition temperature and in similar fashion the cartridges 88 and 88 will be ignited in that women In the event that one of the fuse wires should be broken by the firing of the projectile corre-. spending thereto, the circuit will be completed by the bore hole liquid which fiows into the pewder chamber and grounds the exposed contact. Any other known method may be employed for recompleting the circuit when a fuse wire is broken. This may be accomplished by connecting the firing circuit and gun body to a pair oi o metallic surfaces which are electrically insulated I from one another by a layer of insulating material. The device is disposed in the explosion chamber in such fashion that the insulating material is destroyed by the explosion- Whenthis condition obtains, the two conductor surfaces are brought into engagement, grounding the circuit at that point and insuring a completed circuit even though the fuse wire is broken during firing.

If it is desired to discharge only certain of the cartridges, the circuit may be opened after the desired number of charges have been fired. Any suitable mechanism may be employed to count the charges as they fire. After part of the charges have been fired-the balance or any part thereof may be fired by reclosing the switch 88.

The series circuit shown in Figure 10 may be further modified by providing fuses with graded resistances, as well as graded time delay means. In such case, a second series of fuses could be provided above the series illustrated in Figure 10 which would differ fromthose shown only in that each fuse would require a larger current to heat it to ignition temperature. The current would necessarily have to be increased to fire each successive series of cartridges. Accordingly, a large number of shots could be fired in a single circuit by using several series of fuses of varied resistance and varied time delay characteristics.

In Figure 11 is shown a schematic wiring dia-. gram illustrating a plurality of delayed action fuses connected in parallel which may be selec- 15 tively fired without interrupting the flow of cura switch 94 to the primary winding 95 of a conventional type transformer 06. The secondary winding 91 of the transformer 96 is provided with a variable tap 08 which is grounded at the point 99 at the surface of the earth. The other terminal of the secondary winding 01 of I the transformer 96 is connected through a conductor 99, an ammeter I00, a resistance IN and the conductor 62 to a plurality of dissimilar, parallel-connected fuses I02, I03, I04 and I05, the other ends of which are connected to ground, the intervening earth serving as a return conductor.

The fuses I02, I03, I04 and I05 have similar electrical properties so that the same amount of current will flow through each cartridge and the same amount of heat will be developed. How

ever, the. fuses I02, I03, I04 and I05 are provided with different amounts of heat insulatin material, the fuse I02 having the most and the fuse I05 having the least, or preferably none at all. Hence the fuses I05, I04, I03 and I02 will fire in that order. The sequence can obviously be interrupted at any time by opening the switch 94 and re-established again by reclosing the switch 94. The ammeter I00 will give visual indications of the shots fired because current increments will be observed when each fuse wire is broken due to the firing of .a projectile. It will be noted that the operator need make no adjustments during the shooting sequence.

The embodiment of the invention-illustrated in Figure 11 may be further modified by using I fuses of different electrical resistance with equal time delay periods or a combination of fuses having both graded resistances and graded time delay periods. Such combinations greatly increase the number of shots which may be selectively fired without necessitating abnormally high voltages or currents.

The modifications described above are intended to be illustrative only and the invention is not intended to be limited in any way thereby but is susceptible of numerous changes in form and erties, respectively, so chosen that the delay for each heating element will be different from the.

others, an energizing conductor mounted on the outside of the body, and electrical connections between said conductor and each of said pro- .Iectiles.

3. In apparatus for igniting a plurality of ex- Q plosive charges in a body lowered into a bore hole, the combination of a plurality of seriesconnected heating elements for igniting said explosive charges, means interposed between said respective heating elements and said explosive charges for delaying the transfer of heat to the explosive charges corresponding thereto, the thermal properties of said respective heat transfer delaying means being so chosen as to cause them to reach the ignition temperatures of the corresponding explosive charges at differenttimes, and a source of electrical energy connected to said series of heating elements.

4. In apparatus for igniting a plurality of explosive charges in a body lowered into a bore hole, the combination'of a plurality 01' series connected heating elements for igniting said explodetail within the scope of the appended claims.

elements to the corresponding explosive charges,

said delaying means having different thermal properties whereby the delay will be different for each of said elements.

2. In gun apparatus for use in wells, the combination of a body adapted to be lowered into a bore hole, said body havinga plurality of gun bores therein and powder chambers therefor, an

insulated projectile in each of said gun bores,

an explosive charge in each ofsaid powder chambers, heating elements for igniting said explosive charges. said heating elements each having one terminal grounded to the body and another terminal connected to a projectile, a plurality of heat insulating means for delaying the transfer of heat from said heating elements to the explosive charges corresponding thereto, said insulating means having diife'rentthermai prop sive charges, a plurality of heat insulating members interposed between said heating elements and said explosive charges, each having diil'erent heat transfer characteristics so chosen that the ignition temperatures of the respective explosive charges will be reached at different times, and a source of electrical energy connected to said'series of heating elements.

5. In apparatus for use in bore holes, the combination of a body adapted to be lowered into a bore hole, said body having a plurality of gun bores therein and powder chambers therefor, an explosive charge in each of said powder chambers, electrically connected heating means in each of said powder chambers for igniting the respective explosive charges therein, said respective heating means having diflerent thermal properties so chosen as to cause them to reach the ignition temperatures of "the explosive charges at diilerent times, and a sourceof electrical energy connected to said series-connected heating filaments.

6. In bore hole, apparatus the combination of a body adapted to be lowered into a bore hole, said body having a gun bore therein, an insulated projectile in the gun bore, a resistance winding disposed in a recess in the body near the gun bore. a source of direct current having its positive pole connected to said resistance winding a closure for the recess having a small bore therein, and an insulated conductor extending from said projectile and through the bore in the closure to one of the terminals of said resistance winding, whereby upon firing of said gun, said conductor is disconnected from said winding and an insulating medium is formed at the point of disconnection to reduce current leakage from the conductor.

"I. In a gun perforator, the combination of a body adapted to be lowered into a bore hole, said body having a gun bore therein, an insulated projectile in the gun bore, means forming a groove in the periphery of the body and extend-v ing longitudinally of the body, a conductor in said groove, a resistance winding disposed in a recess in the body near said gun bore and connected in series with said conductor, a source of direct current having its positive pole connected to said conductor, a closure for the recess having a small bore therein, and an insulated connection between said projectile and said resist-- anc winding through the bore in the closure, whereby upon firing of said gun, said conductor is disconnected from said winding and an insulating medium is formed at the point of disconnection to reduce current leakage at said point of sconnection.

8. Ina gun periorator, the combination oi' a body adapted to be lowered into a bore hole, said body having a gun bore therein, an insulated projectile in the gun bore, electrical igniting means connected between said projectile and ground,'means forming a groove in the periphery ofthe body. and extending longitudinally of the bo y. a conductor in. said groove, a resistance winding disposed in a recess in the body near said gun bore and connected in series with said conductor. a closure for the recess having a small bore therein, an insulated connection between said projectile and said resistance winding through the bore in the closure and a source of electrical energy having its positive terminal electrically connected tosaid conductor and its'negative terminal electrically connected to ground.

9. In apparatus for-igniting a plurality of explosive charges in a body lowered into a bore hole, the combination'oi' a plurality or psrsllel connected heating elementsfidr igniting said explosive charges, means'interposed between-said ,respective heating elements and said explosive charges for delayingthe transfer 01' heat to the explosive charges corresponding thereto, the thermal properties of said respective heat transfer delaying means being so chosen as to cause them to reach the ignition temperatures oi the corresponding explosive charges at diilerent times, and a source or electrical energy connected to said parallel heating elements,

HENRI-GEORGES DOLL. 

